Fluid fossil fuels such as oil and natural gas are becoming scarce as these fuels are consumed by a world whose population is continually growing. Thus much attention is being directed towards producing liquid hydrocarbons from solid carbonaceous materials such as coal which is available in abundant quantities in some countries such as the United States.
In U.S. Pat. No. 3,936,233 Sass et al disclose a process for recovery of liquid hydrocarbons from comminuted coal by pyrolyzing the comminuted coal in the presence of a particulate source of heat. The pyrolysis of the coal yields pyrolytic vapors containing volatilized hydrocarbons and char. In this process, char is separated from the pyrolytic vapors and the particulate source of heat for pyrolysis of incoming coal is furnished by oxidation of separated char.
Although processes such as Sass' process are useful for achieving high yields of liquid hydrocarbons from coal, the liquid hydrocarbons resulting from condensation of the pyrolytic vapors can be contaminated by particulate matter. This reduces the economic value of the liquid hydrocarbons. This particulate matter, which is primarily char, is the result of incomplete separation of char fines from the pyrolytic vapors. Char fines result from pyrolysis of coal fines produced by comminution of the coal, reduction of the size of char particles due to oxidation, and attrition of char particles during processing.
Char fines can be present in the liquid hydrocarbons because devices for separating char from pyrolytic vapors are relatively inefficient for fines, especially fines less than about 10 microns in diameter. Once in the condensed hydrocarbons, the char is difficult to remove because the hydrocarbons typically are highly viscous and a large portion of the fines are less than about 10 microns in diameter, thus making their removal by conventional solids separation techniques such as filtration and centrifugation from the hydrocarbons very difficult.
Thus there is a need for a process for recovery of liquid hydrocarbons from solid carbonaceous materials such as coal in which the fines content of the solids introduced into the pyrolysis reactor is minimized to minimize contamination of the liquid hydrocarbons by solids.